1. Field of the Invention
The present invention relates to an evaporation apparatus, and more particularly to an evaporation apparatus which functions as an evaporation source for supplying an evaporation material which is heated and evaporated to a deposition target.
2. Description of Related Art
Conventionally, evaporation (in particular, vacuum evaporation) has been used widely for formation of thin films made of various materials. For example, in organic electroluminescence (hereinafter abbreviated as “EL”) displays, which have attracted attention as a possible replacement for liquid crystal displays and which have been developed for practical use, vacuum evaporation is commonly used for forming an organic thin film and a metal electrode layer used in an emissive layer of an organic EL element of such a display panel.
A vacuum evaporation apparatus includes a crucible having high heat resistance and excellent chemical stability within an evaporation chamber. A deposition material (evaporation material) placed in the crucible is heated and evaporated to thereby form a deposition layer on a deposition target. Conventional vacuum evaporation apparatuses employ a single point-like evaporation source, which discharges the evaporation material in the radial directions toward the deposition target surface for forming a layer thereon.
There is meanwhile continuous demand for displays, including organic EL displays, having ever larger areas. An evaporation apparatus used for an organic EL display must therefore accommodate larger panel substrates on which an element is formed, in other words, the apparatus must accommodate an increased deposition area.
On the other hand, for medium or small size panels, a so-called gang printing technology is often used, in which a plurality of panels are simultaneously formed on a single large substrate (mother substrate) and are separated as individual panels later. For these medium and small panels manufactured by gang printing, reduction of manufacturing cost requires that the size of each mother substrate be increased, to increase the number of panels which can be formed simultaneously. For the manufacture of such panels, as in the large display panels described above, it is necessary to accommodate an enlarged deposition area because evaporation is performed for a large mother substrate.
When a single point evaporation source as described above is used for evaporation with respect to a large area as described above, the distance between the evaporation source to a film forming position significantly varies depending on the position of the deposition target substrate, which hinders formation of a uniform deposition layer on the substrate. To address this problem, Japanese Patent Laid-Open Publication No. 2001-247959, for example, suggests using an elongate evaporation source, which is so-called linear source. Use of such a linear source reduces variations in differences between each position of the substrate and the linear source, thereby enhancing the uniformity of evaporation conditions with respect to a substrate having a large area.
Because any variations in emission brightness and emissive color significantly affect the quality of a display, uniformity of emission brightness or the like is a strong requirement of all displays, including organic EL displays. However, as described above, when manufacturing an organic EL display, an emissive layer, an organic layer such as a charge transport layer and a charge injection layer, and a metal electrode are formed using a vacuum evaporation method. Because an organic layer is a very thin film, any variation in the film thickness has a relatively very large effect on a variation in the emissive brightness and emissive color. Further, because an organic layer is formed between an anode and a cathode, any variation in the thickness of the organic layer has the possibility of creating a display defect such as short-circuit formed between the anode and the cathode. Accordingly, an evaporation apparatus which is used for such an organic EL display or the like, for example, requires that a deposition layer be formed on a large area with very high accuracy.
When a linear source as described above is used for manufacturing an organic EL element, deposition of a film onto a large substrate would be easy. However, even when a linear source is simply used to form an organic layer or the like using evaporation, the characteristics of the resultant organic EL element significantly vary and it is not possible to realize the uniformity required for practical use of an organic EL display.
The applicant of the present invention researched and studied causes of variation in the element characteristics described above and found that a major factor thereof is that discharge of a deposition material is not uniform along the longitudinal direction of an evaporation source when a linear source is used as the evaporation source. In order to form a uniform deposition layer on a wide deposition surface, it is necessary to discharge a deposition material uniformly from all the positions of a linear source in the longitudinal direction.